Heat pumps are well known and extensively used in the heating, ventilation, and air conditioning industries. The most significant advantage to heating and cooling systems employing a heat pump is that they use the same components to effect both heating and cooling operations, as opposed to other systems that require a substantial number of separate equipment components for carrying out both heating and cooling functions. Conventional heat pump systems use a compressor operated by an electric motor to circulate refrigerant through a condenser which converts a gaseous form of the refrigerant to a liquid form of the refrigerant. The liquid refrigerant then passes through an evaporator which either absorbs heat from or imparts heat to an area to be cooled or heated.
Advancements in heat pump system technology have primarily focused on the development of improved working fluids and system components. In the case of working fluids, different refrigerants, and particularly different fluorocarbon compounds, have been developed which have optimum performance characteristics in particular equipment or operating ranges. In regard to the system components, efforts have been made to improve the operation and efficiency of the compressor, condenser, evaporator, and other ancillary components of these systems. However, due to the relatively advanced stage of the technology, only minor improvements in operation and efficiency have been achieved in recent years.
Previous attempts have been made to develop heat driven heat pumps, such as engine driven and absorption type heat pumps. These heat driven heat pumps have not achieved any significant commercial success or acceptance for a variety of reasons. In general, these systems tend to be highly complex and include components that are both sophisticated and expensive. In addition, many of these systems utilize working fluids other than conventional working fluids, such as ammonia or lithium bromide. These alternative working fluids can be potentially hazardous and require new and different procedures and equipment for installation, repair and service.
Heat driven heat pumps employing heat engines and conventional working fluids have been developed but typically suffer from a number of disadvantages. For example, fluid having a high temperature and a high pressure remains in the heat engine cylinder at the end of each power stroke of a conventional heat engine. When this fluid is discharged into a condenser, there is a loss of a significant amount of energy, thereby rendering the system highly inefficient. Another problem in such conventional heat driven heat pumps is the desire to combine the heat engine and compressor within a single housing having a piston rod connecting the heat engine piston with the compressor piston. Since the optimum characteristics for the working fluids in the heat engine and the compressor of these heat pumps are substantially different, it is necessary to employ two different refrigerants. Under these conditions, the higher pressure working fluid eventually migrates through any seals into the lower pressure working fluid, thereby altering the characteristics of the lower pressure working fluid.
Another difficulty in the design of heat engines relates to the valve which effects shifting between the high vapor pressure stroke and exhaust stroke. When a conventional valve shifts, it allows vapor to blast into the cylinder so rapidly that it induces the entire heat pump to vibrate or shake violently, thereby shortening the life of the heat engine. Efforts to slow the valve shift motion have been unsuccessful because the high pressure vapor leaks by the valve when it slowly passes from the high vapor pressure to the exhaust position.
Yet another notorious problem with heat engines is in effecting start-up if the unit has not been running and is cold. When the heat generator or evaporator first begins to deliver hot vapor to the power chamber of the heat engine, it condenses before it can drive the piston. Since there is a limited amount of working fluid, the heat generator can run low on working fluid before the power chamber of the heat engine reaches operating temperature. This causes overheating of the heat generator and possible scorching of the working fluid, thereby requiring major servicing before the heat pump can resume normal operation.
The trucking industry has endeavored for some time to develop an effective and commercially viable system for heating and cooling a truck cab. Of particular concern is the heating and cooling of the sleeper cab for extended periods of time. Conventional practice has been to allow the truck to idle over-night in order to maintain the desired temperature within the cab while the driver sleeps. This approach, while effective, may cause ten or more gallons of diesel fuel to be used by the diesel truck engine in a single night, resulting in an extremely high cost of operation. Increasing regulations on maximum daily driving time in conjunction with increased pressure on drivers to meet tight schedules on long hauls have exasperated the problem. Drivers also face pressures from environmental groups and government agencies to reduce emissions by employing other methods of heating or cooling the truck cabs rather than idling the engine.
Anti-idling alternatives have thus far proven ineffective for various reasons, including undesired increases in the weight of the truck resulting in reduced fuel efficiency, system complexity, and system requirements resulting in time limits on system operation. In addition, current anti-idling technologies require that conventional heating and cooling systems be employed during operation of the truck, resulting in unnecessary energy consumption to maintain the truck cab at a desired temperature while the truck is running.
Thus, the need exists for a heat pump system solving one or more of the disadvantages or limitations discussed above. In particular, there is a need for a mobile heat driven heat pump system that may be employed as an anti-idling technology, in addition to other heating and cooling applications.